In recent years, various nonvolatile semiconductor memory devices have been developed and practically used. One of them is a variable resistance memory such as a ReRAM that utilizes the resistance variation of an oxide (for example, Non-patent Document 1).
The variable resistance memory has a structure of an insulating thin film composed of an oxide, which is sandwiched between two metal electrodes. It is an element capable of exhibiting a resistance variation from the high-resistance state to the low-resistance state, or from the low-resistance state to the high-resistance state, on application of a voltage or current between the electrodes. This reversible resistance variation is used to store data. In the instant document, the variation from the high-resistance state to the low-resistance state is called “set” or “write” and the variation from the low-resistance state to the high-resistance state is called “reset” or “erase”.
Such the variable resistance memory includes the unipolar type that executes current/voltage application for set and reset both in one direction and the bipolar type that executes current/voltage application for set and reset in opposite directions. The former is frequently found in those that use a transition metal oxide of the binary system composed of two elements: a transition metal and oxygen. The latter is frequently found in those that use an oxide of a ternary (2 bits per cell) or higher system composed of three or more elements including oxygen (for example, Non-patent Document 2).
The unipolar type applies a lower voltage at the time of reset than at the time of set, for a longer time than at the time of set, to transit the variable resistor to the high-resistance reset state. In this case, the reset current flows in load resistances such as the driver in the variable resistance memory, the current/voltage source circuit, the parasitic resistance on wiring, and the selected memory cell. In the set state before reset, a large current flows because of the low-resistance state, which though transits to the high-resistance state at the time of reset. Therefore, in relation to other load resistances, the voltage across the variable resistor rises instantaneously. If the voltage across the variable resistor exceeds the set voltage at that time, the variable resistor transits again to the low-resistance state and may cause a problem possibly because it can not be reset.